__I2C__ is a [hardware] bus used across many microprocessors and microcontrollers, including the [Raspberry Pi].
It permits sensors and other IO devices to be connected to the controller (and/or each other) with only four wires (or really only ''two'' if you don't count Vcc and ground): 

# Vcc (3-6v)
# SDA
# SCL
# Gnd (Ground)

The Pimoroni "Breakout Garden" set of I2C sensors also includes an "{{{INT}}}" (interrupt) pin, ''AKA'' "{{{GCLK}}}" or "{{{GPIO4}}}".

Some well known I2C products: 

|| Stock || Brand  || Description                               || 7 bit address         || Notes
| 1 | Pimoroni     | LTR-559 Light & Proximity                  | 0x23                   |
| 1 | Pimoroni     | VL53L1X Time of Flight (ToF)               | 0x29                   | range 40mm-4m
| 1 | Adafruit     | VL6180X Time of Flight Distance Sensor     | 0x29                   | range 5-100mm
| 1 | Pimoroni     | 11x7 LED Matrix #1                         | 0x75                   |
| 1 | Pimoroni     | 11x7 LED Matrix #2                         | 0x77                   |
| 1 | Pimoroni     | 5x5 RGB Matrix                             | 0x74 / 0x77            | 77 will be in conflict with 11x7 #2
| 1 | Pimoroni     | ADS1015 +/-24V ADC 3 Channel               | 0x48 / 0x49            |
| 2 | Adafruit     | ADS1015 12 bit ADC 4 Channel w Prog Gain   | 0x48 - 0x4B (jumpers)  |
| 1 | PiBorg       | Thunderborg                                | 0x10 / 0x11            |
| 1 | PiBorg       | Ultraborg                                  | 0x36                   |
| 1 | Pimoroni     | ICM20948 9-DOF Motion Sensor               | 0x68 / 0x69            |
| 1 | Adafruit     | BNO055 Absolute Orientation Sensor         | 0x28 / 0x29            |
| 0 | Adafruit     | VCNL4010 Proximity Sensor                  | 0x13                   |
| 0 | Adafruit     | LSM9DS1 Accel Gyro Mag Temp 9-DOF          | 0x1C / 0x1E Accel, 0x6A / 0x6B for Gyro |
| 0 | Adafruit     | VL53L0X Time of Flight Distance Sensor     | 0x29                   | range 50-1200mm (2m on long range)

"Stock" is how many [Murray] has in his robot laboratory as part of exploratory design and development of the [KR01] robot.

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[{Tag Hardware}]